Of course it can. By changeing something into something better.
Want a more powerful body? Morph your muscles to have strength per size equal to those of a cat.
Or simply change your muscles to be more powerful...
On a related note, would it simply be a higher Cr Co Ritual to make a Giant Blooded person to permanently go from a +5 to a +6 in Str or Sta? +6 is the top end for giant blooded.
I'd say so - in fact the level for the stat enhancement part would be the same as the +5 for normal humans because giants have a higher average but there would need to be a magnitude for a size +2 target.
Right, forgot about the higher magnitude for size in the first place.
So, if you think altering stats is a reasonable thing for Muto to do, what do you think of the scheme in the OP? Should the degree of change be relative to the species norm as I have set out or to the stat of the individual targeted? Should the base level be higher or lower? How about the interval size?
This is kind of explicitly mentioned in the descriptions of the arts. Namely that if you want to be as fast as the fastest human you use creo, if you want to run faster than the fastest horse you use muto. The problem is that gift of bears fortitude is the only spell that really effects a stat and it is derived stat not a direct stat.
Mmm. My example would also be a derived stat though. Probably something like a straight +2 or +3 to your normal Strength.
Anyway, i think its quite obvious that "Muto can enhance".
I’m attempting to get back to the original question, without having a full bag of answers, unfortunately. As already insinuated, it would not make sense to allow for a spell to increase Stamina by +1 unless it was with a base of level 10 or higher (for soak balancing reasons). Let’s assume that Stamina +1 is base 10, +2 is base 20, +3 is base 30 etc. up to a maximum stat of the given beast simulated. Similarly, I would suggest balancing the soak bonus to the actual soak bonus of a normal bear (although this seems irrelevant as a result of the high spell level). Furthermore, let’s assume you take on some feature of the animal in question; let this feature be more drastic with higher levels of change. Assuming that these changes do not change your original size, the alterations are not affected by the actual mass of the animal – only their ‘distilled traits’ traits are transferred. As a result, we should probably look at the size of the animals we get inspired from and adjust accordingly before judging the stats of the animals (each size step up means +2 str, -1 qui). Applying common sense should do the trick though; players should be encouraged to invent their own versions rather than rely on math. Let all the spells be inspired by animals, e.g.
Strength of the Bear/Lion/Boar/Horse (up to +2)
Stamina of the Bear (up to +4)
Dexterity of the Cat (up to +3)
Quickness of the Stag (up to +4)
, similarly (although a wee bit strange) one might include mental stat spells too, such as:
Perception of the Falcon (up to +3)
Presence of the Lion (up to +0)
Communication of the Dog (up to +0)
Intelligence is obviously skipped, as animals have cunning.
Inventing spells that go the other way, lowering stats, should be possible as well in a similar design. A sample spell could be:
Perception of the Falcon
MuCo(An) 30
Personal, Sun, Ind
Alters the senses of the caster slightly so that his powers of Perception is improved, incurring a +2 bonus to Perception. This change often leaves the eyes with a slightly unerring yellow hue.
(Base 20, +2 Sun)
In most cases I would consider casting an ‘eyes of the cat’ spell variant with the same name, adding +3 to rolls involving vision, but with a level of just 4.
In any case i believe that MuCo should only be used for temporary boosts, otherwise it would be Creo. I do believe, however, that the physical changes resulting from these spells, will make most Magi hesitant to make changes permanent (thus game balance is somewhat kept in check).
Hope this was helpful.
If you are looking for physical stats it's probably easier to turn yourself into a modified animal eg:
Becoming the Ursine Champion MuCo 30
Requisites An, He, Te
R: personal, D: sun, T: individual
This spell turns the caster into a brown bear with hands, speaking voice, and a stable upright stance. The requisites change your worn items to fit your new form - weapons are not changed and those with tight grips such as standard arming swords will be hard to wield (-3 att and def). Take the bear's average physical stats - Str +6, Sta +4, Dex +2, Qik 0 - as modifers to your own, your size increases by +2 and you gain Teeth: Init 0, Atk +3, Def +1, Dam +1. The bear's tough hide and thick fur combine to give it a Protection of +3. Soak and wound levels should be calculated according to your new form. You retain your own virtues and flaws. Social interactions that do not benefit from being scary are at -3 to rolls and anyone with armour over thick fur should be wary of overheating. A piece of bear fur draped over the shoulders as part of the casting can be removed by the caster to end the spell early.
base 10 (change into land animal), +2 sun, +1 size, +1 complexity (hands and voice)
Its Muto, that means you´re not limited by normal maximums.
You DONT have to use changes towards animals though. You can just make it a spell to make a human unnaturally superhuman.
I would make it an easier thing to do it "animalstyle" though.
1] I guess you're right. I just don't like the idea, that a girl with 'the aspect of a bears strength' could be stronger than an actual bear (although i do realize that we are dealing with magic). But size is our balancing friend here, and with the spells being of such a high level, it should not be a problem. I'll agree on allowing the spells to have no maximum based on the animal feature simultated, but I would personally still rule against 'Intelligence of the Owl +3' and 'Presense of the Peacock +2'...
2] I like the fact, that the animals features show slightly. Plus if the animal style was not neccessary, and the minor alterations are not required, i guess the wizard inventing 'Gift of the Bear's Fortitude' was either a moron or a sucker for flavour 
Ill repeat myself, you do NOT have to use any of the animal references. Thats just a simpler way of doing it.
Muto can change a human to superhuman, no animal references needed or involved.
Its probably a good deal easier to do it that way.
Thanks for stating that again 
Please note, that i do not disagree with you.
Rather than forcing you to repeat yourself, could you please humor me (and possibly the author of the OP) with a take on one of these spells, that do NOT use animals references?
The way i see it, the OP was about rules and game mechanics for designing spells with stat boost effects - something players in my troupe have been wanting to do as well. So could you please elaborate on the 'harder way' through a spell, so that we might know what level a spell like that would have.
My basic intention is ~2 magnitudes higher than doing it through animal aspects. Although "Gift of the bears fortitude" kinda breaks it as the "bear" part is only in the name(no An requisite). So i might say that by adding an An requisite to it, its level could be dropped to 20 and probably get rid of the penalty in the process.
I believe i would prefer to start higher and then add 1 Magnitude for each better, but i´m still debating it with myself. Also there is much to be said for making the change relative to the target, not the average of the target creature. As otherwise you´re still retaining the "normal range" that is Creo rather than Muto.
Something like:
20 Change a stat by +-1 , this would mean a level 30 spell to get a +1 for self for a day...
25 Change a stat by +-2
30 Change a stat by +-3
35 Change a stat by +-4
40 Change a stat by +-5
45 Change a stat by +-6
50 Change a stat by +-7
and
For Creo, it states: "Further, a magus can only create something he knows about." While RAW does not stand either way, I prefer to apply this to Muto also.
A magus may know about superhuman strength, in this I agree with your +2 magnitude for not using a natural approach. But you could never Muto your eyes to shoot laser beams as you cannot know any natural or mythical example of laser.
That is only true if you apply your personally added restriction about the need to know.
The core of Muto is exactly that it CAN produce whatever results you want. Anything you can think of, which is why the restriction isnt a good idea for Muto. And AM5 pretty much overrules you as well. Because there´s lots of spells that does things that noone could really be expected to "know of".
For example, ill bet noone would expect fire to turn into ice, but one of the example spells in the main book does exactly that.
So i cant agree with you.
So, if an An requisite makes Co changes easier and presumably a Me requisite helps alter animal's minds, what requisite makes MuMe 2 magnitudes lower?
None for Int as has already been noted. The others, An should be workable... But then the limit is what the Animal used could/should have.
Plenty of animals for perception. Peacocks, songbirds, tigers for presence. What to raise communication?
"... can grant or remove properties something cannot naturally have." Some results are not properties.
{citation please}
You completely missed the boat.
Can you Creo ice? Yes, therefore you can muto anything to grant some ice properties.
Can you Creo laser? No, therefore you can cannot.
You seem to say RAW forbids this interpretation. Show me the money.